I think they probably just need time to settle in but I'd like some opinions to be sure there's nothing else I can do.

We got 2 Angelfish from the local shelter on Saturday (5/21). They are about 5-6" from the tip of their top fin to the tip of their bottom fin. I've put them in a 55g tank which has one tiny Pleco. I started the tank with an established canister filter and confirmed that there was no ammonia to begin with and that there was some Nitrate.

They seem to be adverse to any light - even ambient light in the room, I have the aquarium lights completely off. They basically hover in the corner and then move around (in the corner) a little bit. They sometimes rest sort of crooked. They readily ate some flake food earlier today so at least they have an appetite.

I would have done things differently but this wasn't a planned event - we saw them at the shelter and decided to adopt them. Using an established canister filter from another tank, I thought, would be a good idea because it would hopefully handle the Ammonia conversion, etc. So I was a little surprised to see Ammonia show up on the test.

Keep doing what you're doing and give them a little time to settle in. Just keep an eye on the ammonia and do water changes as needed. Once the mini-cycle is done they will be happier. Angels can be a bit fragile but once settled in they'll be fine. It's a good sign that they are eating. Soon they will be begging at the glass and eating from your hand.

I agree. Do you have any live plants in your tank? My Angelfish seem to hang out around the live plants. They especially seem to like the floating plants. I have Pennywort in my tank and they love that. They like picking/hunting around in that for littlee tidbits of food. Keeps them quite busy.

Excellent. To me, the more plants the better and most fish seem to find comfort in being able to hide in or around them. My Angels are quite active so obviously I have to keep enough room for them to swim around but the plants seem to keep them quite comfortable. The plants will also help with any trace Ammonia that might show up. They might very well just be in a state of shock and need time to settle in to their new and improved digs. :)

They will be fine once settled. I remember this situation from the other thread. The more cover they have, esp now, the better. Angels live in very dimly-lit waters, thick with branches, stumps, etc., and overhanging vegetation. So no light (careful with plants though), or lots of floating plants, will help a lot. And as Inga said, rooted plants are great as they can "hide" among them and feel secure.

I posted this video a while back on angels in the Amazon, here it is again as it illustrates their environment.

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian.[unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If youíre going to take it under your wing then youíre responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]

Update: Their now happily swimming all around the tank. I added a black background which instantly seemed to comfort them. Maybe they felt too "exposed" before. I have an order of plants coming in tomorrow and they are now under full lighting and are behaving well. They come up for food when I'm near the tank already so I think they've settled in well!

I'll continue to monitor the ammonia until the levels even out, I expect to be doing daily or bi-daily water changes until that's the case.